In a large stock pot over medium heat, cook sausage, fennel, onion, bell pepper, and garlic. Heat until sausage is fully cooked and vegetables are tender, about 5 minutes. Drain any fat and wipe out the pot with a paper towel; if the fat won't drip off, I place the meat in a bowl with a paper towel underneath and on top, to soak up the grease.
Return meat mixture to the pot and add the chicken broth, tomato sauce, diced tomatoes, salt, and crushed red pepper. Simmer for 20 minutes with the lid on.
Add broken pieces of lasagna noodles, bring to a boil, and cook 10 to 12 minutes; until tender.
Remove from heat, and add basil, parmesan, and mozzarella cheeses. Stir until cheese melts. Serve hot.
Notes
For a super smooth tomato soup, blend the chicken broth, tomato sauce, diced tomatoes, salt and crushed red pepper together with a blender or in the pot with an immersion blender. Then add the meat and continue cooking as explained.